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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Pavel Begunkov <asml.silence@gmail.com>,
        Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 5.10 07/63] io_uring: close a small race gap for files cancel
Date:   Mon,  4 Jan 2021 16:57:00 +0100
Message-Id: <20210104155709.164282473@linuxfoundation.org>
In-Reply-To: <20210104155708.800470590@linuxfoundation.org>
References: <20210104155708.800470590@linuxfoundation.org>
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From: Pavel Begunkov <asml.silence@gmail.com>

commit dfea9fce29fda6f2f91161677e0e0d9b671bc099 upstream.

The purpose of io_uring_cancel_files() is to wait for all requests
matching ->files to go/be cancelled. We should first drop files of a
request in io_req_drop_files() and only then make it undiscoverable for
io_uring_cancel_files.

First drop, then delete from list. It's ok to leave req->id->files
dangling, because it's not dereferenced by cancellation code, only
compared against. It would potentially go to sleep and be awaken by
following in io_req_drop_files() wake_up().

Fixes: 0f2122045b946 ("io_uring: don't rely on weak ->files references")
Cc: <stable@vger.kernel.org> # 5.5+
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/io_uring.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5861,15 +5861,15 @@ static void io_req_drop_files(struct io_
 	struct io_ring_ctx *ctx = req->ctx;
 	unsigned long flags;
 
+	put_files_struct(req->work.identity->files);
+	put_nsproxy(req->work.identity->nsproxy);
 	spin_lock_irqsave(&ctx->inflight_lock, flags);
 	list_del(&req->inflight_entry);
-	if (waitqueue_active(&ctx->inflight_wait))
-		wake_up(&ctx->inflight_wait);
 	spin_unlock_irqrestore(&ctx->inflight_lock, flags);
 	req->flags &= ~REQ_F_INFLIGHT;
-	put_files_struct(req->work.identity->files);
-	put_nsproxy(req->work.identity->nsproxy);
 	req->work.flags &= ~IO_WQ_WORK_FILES;
+	if (waitqueue_active(&ctx->inflight_wait))
+		wake_up(&ctx->inflight_wait);
 }
 
 static void __io_clean_op(struct io_kiocb *req)